A magnetic resonance imaging apparatus is an imaging apparatus which excites a nuclear spin of a test object put in a static magnetic field by using an RF (Radio Frequency) signal of a Larmor Frequency, and reconstructs an image from a magnetic resonance signal produced from the test object together with the excitation.
Various kinds of fat suppression methods for suppressing a signal coming from fat in a test object (fat signal) as an unwanted signal are known in the field of magnetic resonance imaging. Fat suppression methods ordinarily used in general include a CHESS (chemical shift selective) method, a SPIR (spectral presaturation with inversion recovery) method (called SPECIR as well), a STIR (short TI inversion recovery) method, etc.
Among those methods, the CHESS method is a method for frequency selectively suppressing only a fat signal by using a fact that resonance frequencies of water and fat protons differ from each other by 3.5 ppm. Thus, the CHESS method is called a frequency selective fat suppression method, as well. According to the CHESS method, an RF pulse having a resonance frequency of fat (called a CHESS pulse) is applied to the test object at a flip angle of usually 90 degrees before acquiring data. The application of the CHESS pulse causes only vertical magnetization of fat to fall frequency selectively by 90 degrees. Then, if a gradient magnetic field pulse called a spoiler pulse is applied, horizontal magnetization of fat disperses and disappears. Then, if data acquisition begins after the spoiler pulse is applied, data can be acquired in condition such that a fat signal is suppressed.
The SPIR method is one of frequency selective fat suppression methods which use the difference of resonance frequencies between the water and fat protons, as well. According to the SPIR method as well, an RF pulse having a resonance frequency of fat (called a SPIR pulse) is applied to the test object before acquiring data. A flip angle of the SPIR pulse is usually set between 90 and 180 degrees, though. If the SPIR pulse is applied, vertical magnetization of fat falls frequency selectively by an angle according to the flip angle. As the flip angle of the SPIR pulse is between 90 and 180 degrees, the vertical magnetization of fat turns negative immediately after the application. Then, the vertical magnetization of fat increases as time passes owing to vertical relaxation, and recovers from the negative value, by way of a null point and up to a positive value. A recovery rate of the vertical magnetization is determined by the vertical relaxation (T1 relaxation) of fat. A period of time between the application of the SPIR pulse and the beginning of data acquisition for image reconstruction (more strictly speaking, application of a first excitation pulse for data acquisition) is called inversion time (TI) in the SPIR method. If the above inversion time is made to agree with a period of time since the SPIR pulse was applied until the vertical magnetization crosses the null point in the SPIR method, data can be acquired only from a water signal in condition such that a fat signal is suppressed.
In contrast to the above two imaging methods which are both frequency selective fat suppression methods, the STIR method is a frequency non-selective fat suppression method. The STIR method is an imaging method for suppressing fat by actively using a difference in vertical relaxation time (T1 relaxation time) between fat and water signals, i.e., by using a fact that the vertical relaxation time of the fat signal is shorter than the vertical relaxation time of the water signal. A frequency non-selective pulse (STIR pulse) of a flip angle of 180 degrees is applied to a test object before acquiring data, and this makes vertical magnetization of fat and water protons fall by 180 degrees at the same time so that both are rendered negative. After the STIR pulse is applied, the vertical magnetization of the fat proton recovers in a positive direction and so does that of the water proton. As the vertical relaxation time of the fat signal is shorter than the vertical relaxation time of the water signal, the vertical magnetization of the fat signal first reaches the null point. A period of time between the application of the STIR pulse and the beginning of data acquisition for image reconstruction (more strictly speaking, application of a first excitation pulse for data acquisition) is called inversion time (TI) in the STIR method, similarly as in the SPIR method. If the above inversion time is made to agree with a period of time since the STIR pulse was applied until the vertical magnetization crosses the null point in the STIR method, data can be acquired only from the water signal in condition such that the fat signal is suppressed.
As being a frequency non-selective fat suppression method, the STIR method has an advantage of being hardly affected by unevenness of a static magnetic field. In time of data acquisition, though, vertical magnetization of a water signal (negative value) is smaller than that in a case where no STIR pulse is applied, and thus there is a shortcoming in that an SNR decreases or that it takes a long time for imaging to achieve a specific SNR.
Meanwhile, as the CHESS method is a frequency selective fat suppression method and so is the SPIR method, vertical magnetization of a water signal is not affected by application of a CHESS pulse or a SPIR pulse, causing no SNR decrease differently from the STIR method. As being frequency selective fat suppress ion methods, however, these imaging methods are likely to be affected by a distribution of the magnetic field. If a static magnetic field B0 or an RF magnetic field B1 is spatially uneven, spatial fat distribution after the fat suppression is likely to be uneven.
Thus, an imaging method for enabling fat to be spatially evenly suppressed is proposed for the frequency selective fat suppression method of small SNR decrease. An imaging method for applying an SPIR pulse and further a CHESS pulse, and then starting data acquisition is disclosed, e.g., in Japanese Unexamined Patent Publication No. 2008-264499. As a fat signal having survived the SPIR method is further reduced by the application of a CHESS pulse, the fat signal can be suppressed spatially evenly and to a great extent according to that imaging method.
As fat can be suppressed spatially evenly and to a great extent, the fat suppression method disclosed in JP No. 2008-264499 is quite an effective imaging method from a viewpoint of fat suppression. Meanwhile, fat may be suppressed too much, resulting in inconvenience for image diagnosis in some cases. One example is an image diagnosis of a tumor in a breast, etc., with injection of contrast media. There are lots of mammary glands around the tumor. If there is a tumor, strength of a signal coming from the tumor is usually higher than strength of signals coming from circumferential mammary glands after the contrast media reaches a tumor part. If fat is moderately suppressed, a remaining, spatially broadened fat signal covers lots of mammary gland signals. The mammary gland signals thereby turn inconspicuous, and as a result the signal coming from the tumor can be easily identified.
Meanwhile, if fat is spatially evenly suppressed too much, the mammary gland signals turn conspicuous. As the number of the mammary glands is large in particular, identification of the tumor signal turns difficult because of mammary gland signals around the tumor even if the strength of the signal coming from the tumor is higher than the strength of the signals coming from the mammary glands.
Thus, a magnetic resonance imaging apparatus which does not simply increase an extent of fat suppression but can adjust the extent of suppression to a desirable value is requested.